Gradient side chain grafted anion exchange membranes for fuel cell applications

This study reports on the synthesis, design and evaluation of a novel, interesting type of gradient-side-chain polyphenyl ether anion exchange membranes (AEMs) as the electrolytes for alkali fuel cells. Such membranes were fabricated by co-grafting of quaternized side chains with different lengths onto the polymer backbone. Based on our transmission electron microscopy and small angle X-ray scattering results, the gradient side chain structured AEM can enhance microphase and produce significantly larger ion clusters than that with purely long side chains; this is probably because the gradient side chain structure can possibly reduce the steric hindrance and facilitate cation aggregation during microphase separation. The gradient side chain AEM exhibited enhanced conductivity of hydroxide ions (107.7 mS cm−1 at 80 °C) compared to the uniform-side-chain membrane. It also demonstrated superior alkaline stability, retaining ca. 80% of conductivity and ion exchange capacity after a harsh alkali treatment at 80 °C in 1 M NaOH for 552 hours. Our study preliminarily demonstrates that gradient side chain structure is advantageous over conventional uniform side chains in promoting microphase separation of AEM, and deserves further investigations to optimize the gradient structure for better membrane performance.